Ryan Ainsworth and Grant Seaver, both 13, were students at Treasure Mountain Junior High School in Park City, Utah. They were best friends who shared the hobbies of adventurous Utah teenagers — spending afternoons on skis, dirt bikes or skateboards. Relatives spoke of their bright and beautiful smiles. “Grant was a happy boy,” as Lisa Sippel described her nephew to Utah’s Fox 13, “who lived every minute of his short life to the fullest.”

In September, the young teens died within 48 hours of each other.

The cause was the same.

On Thursday, the Utah Medical Examiner’s Office announced the results of toxicology tests: Acute drug intoxication was to blame, Park City Police Chief Wade Carpenter said in a statement. The chemical culprit was an opioid so obscure it did not have an official name, just a designation, U-47700.

It did, however, have a nickname. Called “pink,” “pinky” or “pinkie,” these were not a reference to its color. U-47700 is a chalky white powder, like crushed Tylenol, unless it is dyed.

“Narcotics users will grow out their pinky finger and take it in their nose, so that’s why it’s called pinky,” Sgt. DeeAnn Servey of the Davis County Sheriff’s Office told Utah’s KTVX News in September.

Until recently, the synthetic opioid U-47700 was not much more than a curiosity, one of many dead-ends in the hunt for superior painkillers. Jacob Szmuszkovicz, a chemist at the Michigan-based Upjohn Company, derived U-47700 from morphine (the “U” taken from Upjohn). But, like fentanyl and other synthetic opioids, U-47700 was a more potent compound. Animal tests indicated the chemical was about eight times stronger than morphine.

And like many synthesized cousins of morphine, if experimental tests of the drug existed they were limited. U-47700 spent most of its prior existence confined to a 1978 patent and a handful of scientific papers. The Food and Drug Administration did not approve it for human use. Doctors at the University of California at San Diego reported in July that they were unable to find pharmacokinetic information — basic details like drug absorption and excretion rates — or therapeutic human data in the medical literature.

In December 2012, U-47700 took a deadly turn, with the first report that it claimed a life — of someone in Norway. The transition from obscure patent to designer drug to death agent followed a path taken by other synthetic opioids. As Katie Mettler reported at The Washington Post in April, W-18 — another powerful opioid — did not exist on the street until “a Chinese chemist found it.” Afterward, “labs halfway around the world started developing the drug for consumers in search of a cheap and legal high.” U-47700 appeared to have fit this global pattern, traveling some 6,800 miles to arrive in Utah.

The deaths of the 13-year-old boys rocked the Park City community. A fellow 15-year-old classmate attempted to commit suicide. Police searched school lockers with K9 dogs. At the time, Carpenter told Fox 13 that police had “received a lot of different social media hits during our investigating” that indicated ” ‘pink’ could be, could possibly be, a cause.”

It was. The police later traced the drug to China. The teenagers had ordered it online, according to unsealed search warrants, and had shipped it to a friend’s house from Shanghai, KSL Newsradio reported. The affidavit stated the delivery held “a clear bag containing a white powder substance.” At the time, U-47700 was legal to own in the United States.

That is no longer the case. The Drug Enforcement Administration declared in September that U-47700 would become a Schedule I controlled substance, saying it was “necessary to avoid an imminent hazard to the public safety.” A temporary scheduling went into effect Oct. 7.

The DEA determined the drug had “no currently accepted medical use and a high potential for abuse,” joining other Schedule I chemicals such as heroin, LSD and marijuana.

Beginning in 2015 and into 2016, deaths from U-47700 cropped up around the country. The DEA reported 15 confirmed fatalities; other estimates are as high as 50 to 120. It has appeared in Florida, New Hampshire, Ohio, Texas and Wisconsin. A Toledo man died in May. A 19-year-old died in a Detroit suburb in October. The Associated Press reported that investigators discovered U-47700 in pills at Prince’s estate.

“The population likely to abuse U-47700 appears to overlap with the populations abusing prescription opioid analgesics and heroin, as evidenced by drug use history documented in U-47700 fatal overdose cases,” the DEA noted in its notice of intent to schedule the drug.

Health-care workers were alarmed by the drug’s toxicity. “It’s causing psychotic disorders like we’ve never seen before,” Tommy Lloyd, program director at Springs Recovery Center in Florida, told CNN affiliate WESH in October. “It’s causing people to completely lose a sense of reality.”

Park City Police Captain Phil Kirk said that the drug can cause a sense of euphoria, but also numbness, sedation and slow breathing, CNN reported.

The police department concluded its Thursday statement with descriptions of the drug. Although typically a powder, it may exist in liquid form, delivered in dropper bottles and nasal inhalers. It is shipped in what the officials called “stealth” delivery boxes, often postmarked from Asia. Inside, containers — boxes, vials or plastic baggies — may be labeled “Not for Human Consumption” or “For Research Purposes Only.”

The officers also had a warning. “U-47700 is extremely toxic, even in small doses,” the police department said. “Exposure to U-47700 by inhalation or contact with skin can be fatal. If you believe you have encountered the drug, contact your local law enforcement agency immediately.”